Secret Intelligence Service vs Great Britain

Secret Intelligence Service and Great Britain both are nouns.

Word	Noun	Adjective	Verb	Adverb
Secret Intelligence Service	Yes	No	No	No
Great Britain	Yes	No	No	No

Secret Intelligence Service (noun)	Great Britain (noun)
the government agency in the United Kingdom that is responsible for internal security and counterintelligence overseas	an island comprising England and Scotland and Wales
	a monarchy in northwestern Europe occupying most of the British Isles; divided into England and Scotland and Wales and Northern Ireland; `Great Britain' is often used loosely to refer to the United Kingdom